A url to notify when indexing is completed.
It should be a POST url and VideoIndexer will add 2 additional query string parameters to it: id and state.

For example, if the callback url is 'https://test.com/notifyme?projectName=MyProject', the notification will be sent with additional parameters to 'https://test.com/notifyme?projectName=MyProject&id=1234abcd&state=Processed'.